Deep look electromagnetic detection and imaging transceiver (EDIT) system for detecting underground passageways

ABSTRACT

An underground tunnel detection system does not employ radar. Instead, an automatic frequency control is used to keep a continuous wave (CW) transmission tuned to the resonant frequency of a resonant microwave patch antenna (RMPA). Changes in loading and the bulk dielectric constant of the mixed media in front of the RMPA will affect its resonant frequency and input impedance. A significant shift in the measured input impedance is interpreted as a possible tunnel, and the phase angle of the measured input impedance tends to indicate a slightly forward or aft position relative to the detection system rolling over it on the ground surface.

FIELD OF THE INVENTION

The present invention relates to electronic sensing devices, and more particularly to sensors and systems that can detect and characterize underground passageways.

DESCRIPTION OF THE PRIOR ART

Every new advance in border patrol, security, and monitoring seems to be met sooner or later with new counter measures and changes in strategy by the criminals. New controls on surface pedestrian and vehicle traffic have been so effective that a larger part of the illegal traffic has been driven underground. Existing pipes, drains, conduits, and tunnels are now being commandeered at accelerated rates, and new tunnels have always been a problem because they are undocumented and hard to locate. Such tunnels are going much deeper into the ground and run farther across the borders. Better technologies are being employed to build these tunnels, and traffic volumes are going up dramatically.

In U.S. Pat. No. 6,473,025, issued Oct. 29, 2002, and titled, LANDMINE LOCATING SYSTEM, the present Inventors describe a ground-penetrating radar that can look into shallow earth to find and identify landmines. Such landmines are necessarily buried shallow so they can be triggered by touch sensors on the surface, and so that the explosives are not blanketed and muffled by too much earth. But sensing other deeper anomalies in the earth like tunnels is a far more difficult problem that needs an entirely new approach to the use of resonant microwave patch antennas (RMPA) as the primary sensor device.

There is a need for a portable device and system that can electronically detect and characterize underground tunnels and passageways that could be used to smuggle people and materials across international borders.

SUMMARY OF THE INVENTION

Briefly, an underground tunnel detection system embodiment of the present invention does not employ radar. Instead, an automatic frequency control is used to keep a continuous wave (CW) transmission tuned to the resonant frequency of a resonant microwave patch antenna (RMPA). Changes in loading and the bulk dielectric constant of the mixed media in front of the RMPA will affect its resonant frequency and the input impedance. Rolling over the ground, a significant shift in the measured input impedance of a downward pointed RMPA is interpreted as a possible tunnel, and the phase angle of the measured input impedance tends to indicate a slightly forward or aft position relative to the detection system on the ground surface.

These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred SPS receivers which are illustrated in the various drawing figures.

IN THE DRAWINGS

FIG. 1 is a time series diagram of a system embodiment of the present invention shown as it is rolled along the ground surface over a buried, underground tunnel;

FIG. 2 is a functional block diagram of the system of FIG. 1 showing how a downward pointed RMPA is used to obtain antenna input impedance measurements that can reveal the presence of a buried, underground tunnel;

FIG. 3 is a perspective diagram of an RMPA as used in FIGS. 1-2 and showing how the relationship of the feedpoint to the groundpost give some phase asymmetry to the effects of tunnels on the antenna input impedance measurements;

FIG. 4 is a cross sectional diagram of RMPA antennas like those in FIG. 1-3 and shows the relationships between the feedpoint, groundpost, fringing E-field, and the propagating E-field;

FIG. 5 is a graph diagram showing how the phase and magnitude of the input impedance of an RMPA can change as it is rolled on the surface over an underground passageway or tunnel;

FIGS. 6A-6C are diagrams representing the RMPA as a loaded transmission line model. FIG. 6A shows the case where the RMPA radiates into air, FIG. 6B shows the case where the RMPA radiates into air and then into the earth, and FIG. 6C shows the case where the RMPA radiates into air, down through the earth and into a tunnel filled with air; and

FIG. 7 is a flowchart diagram of a method embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the present invention do not employ radar. They do not depend on the receipt of an echo signal and the measurement of the roundtrip time the echo signal took to return after the original pulse was transmitted.

Instead, embodiments of the present invention include automatic frequency control to keep a continuous wave (CW) transmission tuned to the resonant frequency of a resonant microwave patch antenna (RMPA). Changes in loading and the bulk dielectric constant of the mixed media in front of the RMPA will affect the resonant frequency and the input impedance.

In effect, the embodiments electronically measure the bulk dielectric constant of a plug of earth directly under a downward pointing RMPA. If such plug of earth has no large air-filled voids, like a tunnel, then one result will be obtained. But if there is a substantial air-filled void, the bulk dielectric measurement of a plug of earth that includes it will be significantly different. If these contrasting differences track as linear segments projected to the ground surface, they can be interpreted as an indication that a tunnel exists. When RMPA's with resonant frequencies of 150-MHz are used, the useful sensitivity of such a system to locate tunnels can reach thirty feet or more into the earth.

FIG. 1 illustrates five different positions of a portable tunnel detection system embodiment of the present invention, and each position as it moves along a path over an underground passageway is referred to herein by the general reference numerals 101-105. In a first position, position-A, portable tunnel detection system 101 directionally transmits a continuous wave (CW) radio transmission from an RMPA into the ground 110 at about 150-MHz. The soil plug below will have a particular dielectric constant, ∈₁, that will produce a load impedance Z₁. Such load impedance Z₁ will influence the input impedance Z_(in) of the RMPA, and changes in the Z_(in) will affect the resonant frequency of the RMPA. If the RMPA is brought back into resonance by fine tuning the frequency of the CW drive, a new minimum Z_(in) will be obtained. That new minimum Z_(in) is represented by impedance vector 111.

For purposes of this explanation only, we can assume the real resistance (R) and imaginary (i) components of Z_(in) to be equal and the vector 111 has a 45° phase angle.

In a second position, position-B, portable tunnel detection system 102 is still transmitting the directional CW radio transmission as before into the ground 110, but now a forward fringe just catches a tunnel 120 filled with air. The air has a particular dielectric constant, ∈₂, that will produce a load impedance Z₂. Such load impedance Z₂ will mix with load impedance Z₁ and together they will influence the input impedance Z_(in) of the RMPA. Again, changes in the Z_(in) will affect the resonant frequency of the RMPA, so it is brought back into resonance by fine tuning the frequency of the CW drive, a second new minimum Z_(in) will be obtained. That new minimum Z_(in) is represented by impedance vector 112.

For purposes of this explanation only, the real resistance (R) and imaginary (i) components of Z_(in) have changed such that the vector 112 has a 50° phase angle. The RMPA is constructed and oriented such that the relative forward or aft placement of loads in its radiation field will affect the phase angle of Z_(in) differently.

In a third position, position-C, portable tunnel detection system 103 is still transmitting the directional CW radio transmission as before into the ground 110, but now the main beam of its radiation field squarely catches tunnel 120. The highly contrasting dielectric constant, ∈₂, will produce a maximum load impedance Z₂ to mix with load impedance Z₁. The effect will be a new maximum in the input impedance Z_(in) of the RMPA. The changes in the resonant frequency of the RMPA are brought back into resonance by fine tuning the frequency of the CW drive, and a third new minimum Z_(in) will be obtained. That new minimum Z_(in) is represented by impedance vector 113.

For purposes of this explanation only, the real resistance (R) and imaginary (i) components of Z_(in) have grown, and vector 113 has a 45° phase angle. This maximum in Z_(in) and minimum in phase angle change from idle conditions indicates tunnel 120 is directly below.

In a fourth position, position-D, portable tunnel detection system 104 continues to transmit the directional CW radio transmission as before into the ground 110, but now the main beam of its radiation field has past tunnel 120 and only a fringe is catching it. The tunnel 120 is now slightly aft. The contrasting dielectric constant, ∈₂, that produces a load impedance Z₂ will be proportionately less in the mix with load impedance Z₁. The sum will produce a new input impedance Z_(in) in the RMPA represented by impedance vector 114. The changes in the resonant frequency of the RMPA are brought back into resonance by fine tuning the frequency of the CW drive, and a fourth new minimum Z_(in) will be obtained.

For purposes of this explanation only, the real resistance (R) and imaginary (i) components of Z_(in) have changed again, such that vector 114 has a 40° phase angle because of the nature of the construction of the RMPA. This past-the-peak measurement in Z_(in) and a telltale phase angle change from previous measurements indicates tunnel 120 is now aft of the RMPA.

In a fifth position, position-D, portable tunnel detection system 105 continues to transmit the directional CW radio transmission as before into the ground 110, but now the main beam of its radiation field is well past tunnel 120. The contrasting dielectric constant, ∈₂, that produces a load impedance Z₂ will drop out of the mix with load impedance Z₁. The sum will produce an input impedance Z_(in) in the RMPA represented by impedance vector 115. The changes in the resonant frequency of the RMPA are brought back into resonance by fine tuning the frequency of the CW drive, and a fifth new minimum Z_(in) will be obtained that more or less duplicates that of position-A.

For purposes of this explanation only, the real resistance (R) and imaginary (i) components of Z_(in) have changed again, such that vector 114 has a 40° phase angle. This past-the-peak measurement in Z_(in) and a telltale phase angle change from previous measurements indicates tunnel 120 is now aft of the RMPA.

FIG. 2 represents an electronics package 200 that can be used in portable tunnel detection systems 101-105. The electronics package 200 includes an RMPA 202 for directionally transmitting a CW signal of about 150-MHz down into the earth 204 to find an underground passageway or tunnel 206. The RMPA 202 will be loaded by the earth 204 and air in tunnel 206 and its input impedance Z_(in) will be affected. The input impedance Z_(in) is constantly monitored by a measurement device 208 and a control signal is sent to an automatic frequency control (AFC) 210. The AFC 210 varies its output frequency in a constant hunt to find the then minimum possible in Z_(in). That then minimum possible in Z_(in) will be at maximum when RMPA 202 is directly over tunnel 206 and tunnel 206 is shallow enough to be an influence on Z_(in).

The measurement device 208 produces two components for each new minimum in Z_(in), a phase measurement 212 and a magnitude measurement 214. These are analyzed by a microcomputer analyzer 216 and compared to one another and characteristic electronic signatures for tunnels. A decision is made whether the current phase measurement 212 and magnitude measurement 214 fit the electronic profile of a tunnel or other passageway filled with air. If so, a tunnel indication 218 is issued as a message to a remote site, a local screen display, and or a sound alarm. The event is stored in a memory 220 with a location tag obtained for the present map position from a global positioning system (GPS) navigation receiver 222. A collection of such events and locations fetched from memory 220 then makes a more informative display possible on a map display screen 224.

FIG. 3 represents a way to construct a RMPA 300 using common FR4 printed circuit board material. A copper-foil backplane 302 and radiating patch 304 are separated by an epoxy substrate 306. A feedpoint 308 is drilled through the backplane 302 and substrate 306 so a 50-ohm coaxial cable can be attached to the radiating patch 304. A groundpost 312 is constructed by drilling and plating a copper via. The relationship of the feedpoint 308 to the groundpost 312 creates a forward radiating edge 314 and an aft radiating edge 316. The RMPA has a characteristic Zin and resonant frequency that are a function of the dielectric constant of substrate 306, objects in the radiated field, the separation distance of backplane 302 and patch 304, the distance between feedpoint 308 to the groundpost 312, and the plan dimension of patch 304. In the embodiments of FIGS. 1-2, these all add up to a resonant frequency of about 150-MHz and a Z_(in) of about 50-ohms when the radiation field is substantially comprised of air. Varactors or other types of trimming capacitors can be added around the edges of RMPA 300 to fine tune its resonant frequency.

Scattering parameters (s-parameters) describe the scattering and reflection of traveling waves when a network is inserted into a transmission line. Here, the transmission line includes the soils and the air inside a tunnel. S-parameters are normally used to characterize high frequency networks, and are measured as a function of frequency. See, National Instruments explanation at, zone.ni.com/devzone/cda/tut/p/id/2513. So frequency is implied and complex gain and phase assumed. The incident waves are designated by the letter a_(n), where n is the port number of the network. For each port, the incident (applied) and reflected waves are measured. The reflected wave is designed by b_(n), where n is the port number. When the incident wave travels through a network, its gain and phase are changed by the scattering parameter. For example, when wave a₁ travels through a network, the output value of the network is simply the value of the wave multiplied by the relevant S-parameter. S-parameters can be considered as the gain of the network, and the subscripts denote the port numbers. The ratio of the output of port-2 to the incident wave on port-1 is designated S₂₁. Likewise, for reflected waves, the signal comes in and out of the same port, hence the S-parameter for the input reflection is designated S.

For a two-port network with matched loads:

S₁₁ is the reflection coefficient of the input;

S₂₂ is the reflection coefficient of the output;

S₂₁ is the forward transmission gain; and

S₁₂ is the reverse transmission gain from the output to the input.

S-parameters can be converted to impedance by taking the ratio of (1+S₁₁) to (1−S₁₁) and multiplying the result by the characteristic impedance, e.g., 50-ohms or 75-ohms. A Smith chart can be used to convert between impedance and S-parameters.

The frequency and impedance, or reflection coefficient (S₁₁), of RMPA 300 are measured to provide sensor information and interpretive reports. RMPA 300 is electronically tuned by a sensor controller either adjusting oscillator frequency and/or varactors to find the resonant frequency of the RMPA each time a measurement is taken. The S₁₁ (reflection coefficient) parameter is measured in terms of magnitude. The sensor controller seeks to minimize the magnitude of S₁₁, meaning RMPA 300 is near its resonant point and 50-ohms.

During an automatic steady state calibration, an iterative process is used in which a sensor controller seeks a minimum in S₁₁ by adjusting the applied frequency through an oscillator. Once a frequency minimum for S₁₁ is found, sensor controller adjusts a bias voltage on varactors connected to the edges of RMPA 300. The voltage variable capacitances of varactors are used to fine tune RMPA 300 into resonance, and this action helps drive the impedance as close to 50-ohms as possible. Sensor controller simply measures the S₁₁ magnitude minimum. Once voltage adjustments to varactors find a minimum in S₁₁ magnitude, the process is repeated with very fine adjustment steps in an automatic frequency control to find an even better minimum. The voltages to varactors are once again finely adjusted to optimize the minimum.

After calibration, an independent shift away from such minimum in S₁₁ magnitude means an air volume is affecting the balance. The reflection coefficient (S₁₁) will change away from the original “calibrated” resonance value. Typically a buried tunnel passing within the field will cause a peak maximum in the measured data. The rate of change of the measured signal in the area is directly related to the speed of the vehicle carrying RMPA 300.

S₁₁ has both magnitude and phase, a real and imaginary part. Changes in magnitude indicate a disturbance in the EM-field of RMPA 300, and changes in the phase provide the directionality of travel 110-113. RMPA 300 is a linearly polarized antenna, the fields on one edge of RMPA 300 are 180-degrees out of phase from the field on the other edge. With a proper alignment of RMPA 300 in situ, people passing in front of RMPA 300 from left to right, for example in FIG. 1, will produce a phase signature that is 180-degrees out of phase from other people moving right to left. The phase at resonance can be corrected to provide a constant 180-degree shift.

FIG. 4 schematically represents an RMPA 400 like those in FIGS. 1-3. RMPA 400 is connected to an electronics controller 402 through a feedpoint 404 with a characteristic impedance of 50-ohms. Such feedpoint 404 passes through a small opening, e.g., on a copper-foil backplane 406 on a fiberglass-epoxy FR4 substrate 408. A copper-foil patch 410 is deposited on the substrate 408 and has a ground connection to the backplane 406 through a ground post 412. The view of FIG. 4 is taken through a normal plane that longitudinally bisects both the ground post 412 and feedpoint 404. A varactor 414 is typical of many that can be connected to be voltage-controlled by electronics controller 402 to enable fine tuning of the resonant frequency of RMPA 400 to help with calibration and measurement sensing. The electronics controller 402 is able to measure parameter S₁₁ at the feedpoint 404 and issue interpretive reports to users.

At resonance, the electromagnetic fields radiate away from RMPA, as shown in FIG. 4. A linearly polarized electric field fringes from the edges of the metalized, copper foil parts of RMPA 400. Such type of polarization is an important operational element of system 100 of FIG. 1, this polarization enables an indication of forward or aft for any tunnels being detected. As applied here, the antenna radiation pattern has a very broad 3-dB beam width of ±30 degrees from the perpendicular to the plane of patch 410. This pattern is important in the present applications because the wide antenna pattern allows a large area to be electronically swept.

FIG. 5 represents an exemplary case in which tunnel 120 is passed over by system 101-105 (FIG. 1) from out of range, then into range, then directly under the RMPA (e.g., 202 in FIG. 2), and then out the other side out of range. For example, a transit time of ten seconds. Such transition begins out-of-range at time=0, the tunnel passes under the RMPA at time=5, and goes out-of-range on the other side at time=10. A change in loading is caused by the changing positions and nearness of the tunnel 120 from time=0 to time=10, and the resonant frequency of the RMPA is affected. So each new resonant frequency is found by controlling the frequency with AFC 210 to produce a minimum in the input impedance of the RMPA.

That new impedance is the measurement that is collected at that particular time, and a series of such measurements is represented in FIG. 5. The ± phase of the impedance-Z tells which edge of the RMPA the tunnel is on, and the magnitude of impedance-Z tells how near the tunnel is. A peak in the magnitude will also be proportional to how large the tunnel is.

Equation (5) and the associated definitions of the variables therein indicate that the RMPA is actually making a bulk dielectric constant measurement. This performance aspect is important because, even though the transmission model relies on the theory of small reflections to balance the system, the bulk dielectric measurement concept eliminates the need for any analysis requiring reflection phenomena, which is a characteristic of radar-based systems.

FIG. 6A represents a side-view of a RMPA 600 radiating into soils or earth 601. A transmission line model can be used to describe the problem, and such are included at the bottoms of FIG. 6A-6C. An RMPA electronics package 602 is configured to monitor the resonant frequency (f_(o)) and the resonant impedance (Z_(o)) parameters of RMPA 600. The resonant frequency is approximated by,

$\begin{matrix} {{f_{o} = \frac{kc}{2\pi\; a\sqrt{ɛ_{eff}}}},} & (1) \end{matrix}$ where,

-   -   k is a constant based on the RMPA operational mode (TM₁₁);     -   c is the speed of light;     -   a is the aperture area of the RMPA; and     -   ∈_(eff) is the effective dielectric constant of the material in         front of the RMPA.

The resonant impedance of RMPA 600 will be,

$\begin{matrix} {{Z_{in} = {Z_{o}\left\lbrack \frac{Z_{2} + {j\; Z_{o}\tan\;\beta\; L_{2}}}{Z_{o} + {j\; Z_{2}\tan\;\beta\; L_{2}}} \right\rbrack}},} & (2) \end{matrix}$ where,

Z_(o) is the impedance of the RMPA (e.g., 50Ω);

$Z_{2} = \sqrt{\frac{\mu_{2}}{ɛ_{2}}}$ is the wave impedance in the media;

μ₂ is the magnetic permeability and for almost all cases μ₂=1;

∈₂ is the dielectric constant of the media;

$\beta = \frac{2\pi}{\lambda}$ is the phase constant in the media;

$\lambda = \frac{c}{f_{O}\sqrt{ɛ_{2}}}$ is the wavelength of the signal in the media; and

L₂=the lateral distance away from the RMPA.

For Equations (1) and (2), the resonant frequency and impedance are directly dependent upon the effective dielectric constant of the media.

FIG. 6B takes the next step, RMPA 600 is suspended over the ground and an air gap 604 intervenes with the earth 601 below. In this case, the equations for the resonant frequency and impedance differ from Equations (1) and (2) in that the effective dielectric constant of Equation (1) becomes instead,

$\begin{matrix} {{ɛ_{eff} \propto \frac{ɛ_{{eff}\; 1}ɛ_{{eff}\; 2}}{ɛ_{{eff}\; 1} + ɛ_{{eff}\; 2}}},} & (3) \end{matrix}$ where,

∈_(eff1) is the effective dielectric constant as given in FIG. 6A, and,

$ɛ_{{eff}\; 2} = {\frac{ɛ_{r} + ɛ_{1}}{2} + {\frac{ɛ_{r} - ɛ_{1}}{2}{\left\lfloor \frac{1}{\sqrt{1 + {12/a}}} \right\rfloor.}}}$ The resonant impedance of RMPA 600 shown in FIG. 6B now takes the form,

$\begin{matrix} {{Z_{in} = {Z_{O}\left\lbrack \frac{Z_{l} + {j\; Z_{O}\mspace{14mu}\tan\mspace{14mu}\beta\; L_{1}}}{Z_{O} + {j\; Z_{l}\mspace{14mu}\tan\mspace{14mu}{\beta L}_{1}}} \right\rbrack}},} & (4) \end{matrix}$ where,

Z_(o) is the impedance of the RMPA (e.g., 50Ω);

L₁ is the thickness of the air gap 604;

${Z_{l} = {Z_{1}\left\lbrack \frac{Z_{2} + {j\; Z_{1}\mspace{14mu}{\tan\left( {\beta\; L_{2}} \right)}}}{Z_{1} + {j\; Z_{2}\mspace{14mu}{\tan\left( {\beta\; L_{2}} \right)}}} \right\rbrack}};$ ${Z_{1} = \sqrt{\frac{\mu_{1}}{ɛ_{1}}}};$ ${Z_{2} = \sqrt{\frac{\mu_{2}}{ɛ_{2}}}};$ and

L₂=the lateral, linear depth dimension of the earth being scanned.

There are direct relationships with the effective dielectric constant for both the resonant frequency and the resonant impedance. Any changes appearing in the dielectric constant will be reflected mainly in changes in the resonant parameters.

FIG. 6B represents the steady state conditions for which electronics 602 would calibrate automatically to RMPA 600.

FIG. 6C extends FIG. 6B by introducing a tunnel 608 into earth 601 within the detection range of RMPA 600. The resonant frequency is still provided by Equation (1), but the effective dielectric constant will now be,

$\begin{matrix} {{\frac{1}{ɛ_{eff}} = {\frac{1}{ɛ_{{eff}\; 1}} + \frac{1}{ɛ_{{eff}\; 2}} + \frac{1}{ɛ_{{eff}\; 3}}}},{where},{{ɛ_{{eff}\; 3} = {\frac{ɛ_{2} + ɛ_{3}}{2} + {\frac{ɛ_{2} - ɛ_{3}}{2}\left\lfloor \frac{1}{\sqrt{1 + {12/a_{3}}}} \right\rfloor}}};}} & (5) \end{matrix}$

Here, a₃ is the size of the RMPA aperture at a position L₁+L₂ away from RMPA 600;

${ɛ_{{eff}\; 2} = {\frac{ɛ_{1} + ɛ_{2}}{2} + {\frac{ɛ_{1} - ɛ_{2}}{2}\left\lfloor \frac{1}{\sqrt{1 + {12/a_{2}}}} \right\rfloor}}};$

And, a₂ is the size of the RMPA aperture at a position L₁ away from RMPA 600; and

$ɛ_{{eff}\; 1} = {\frac{ɛ_{r} + ɛ_{1}}{2} + {\frac{ɛ_{r} - ɛ_{1}}{2}{\left\lfloor \frac{1}{\sqrt{1 + {12/a}}} \right\rfloor.}}}$ For the case shown in FIG. 6C, the resonant impedance of RMPA 600 will be,

$\begin{matrix} {{Z_{in} = \frac{Z_{O}Z_{B}}{Z_{O} + Z_{B}}},} & (6) \end{matrix}$ where,

Z_(o) is the impedance of the RMPA (e.g., 50Ω);

${Z_{B} = {Z_{1}\left\lbrack \frac{{Z_{A} + {j\; Z_{1}\mspace{11mu}{\tan\left( {\beta\; L_{1}} \right)}}}\;}{Z_{1} + {j\; Z_{A}\mspace{14mu}{\tan\left( {\beta\; L_{1}} \right)}}} \right\rbrack}};$

L₁ is the thickness of the air gap;

$Z_{1} = \sqrt{\frac{\mu_{1}}{ɛ_{1}}}$ is the wave impedance in air;

${Z_{A} = {Z_{2}\left\lbrack \frac{Z_{3} + {j\; Z_{2}\mspace{14mu}{\tan\left( {\beta\; L_{2}} \right)}}}{Z_{2} + {j\; Z_{3}\mspace{14mu}{\tan\left( {\beta\; L_{2}} \right)}}} \right\rbrack}};$

L₂ is the distance to the tunnel from the air gap;

Z₂ is the wave impedance in earth; and

$Z_{3} = \sqrt{\frac{\mu_{3}}{ɛ_{3}}}$ is the wave impedance of the air in the tunnel. The tunnel air 606 appears as a very high impedance path for signals from RMPA 600. It is a virtual open, which means that less of the energy of the RMPA signal will be reflected back to the source. Therefore, a RMPA-based sensor system can be employed as a highly sensitive detector of air voids, such as in tunnels.

The electronics 602 constantly measures the resonant frequency and the resonant impedance of RMPA 600. A large air volume passing under the RMPA sensor will dramatically change these characteristics. Since the impedance is a complex number, having a distinct magnitude and phase, the directionality of motion can be determined.

In conventional radar applications, a relatively large signal is transmitted with the hope of measuring a relatively faint return signal. One difficulty in operating a radar type system in a subsurface infrastructure environment is a small signal of interest has to be extracted from many, often stronger reflected signals arriving at the receiver. Radar systems are effectively blind in applications in which the return signal from a first interface can swamp the receiver, leaving the radar unable to detect anything else that may be near the radar antenna.

Embodiments of the present invention do not have such limitations. The RMPA's here and their associated electronics are not used to detect small signals in the presence of very large signals. Instead, the RMPA's are used to make repetitive and continuous measurements of changes in the bulk dielectric constant as targets of interest pass under.

The physical size of the RMPA needed to match operation at resonance can vary quite a lot over the range of 50-MHz to 300-MHz. The size of RMPA 202, 300, and 600 also depends on the dielectric constant of the substrate materials used in constructing it. Typically, the dimension of the metalized surface of the RMPA can be approximated as one quarter (¼) wavelength (λ) in the dielectric. Roughly, D=λ/4*1/√∈, where λ=c/f, ∈ is the dielectric constant of the substrate material, c is the speed of light, and f is the frequency.

The higher the frequency, the smaller physically will be the RMPA. The higher the dielectric constant of the substrate material used in the RMPA, the smaller the RMPA has to be for resonant operation. However, at higher frequencies, the depth of penetration will be reduced because of attenuation. And, at higher dielectric constants for the substrate materials used, RMPA becomes less efficient and has much higher Q. So the right balance will be dictated by the particular application will usually have to be empirically determined. In a prototype system that was built, the RMPA had to resonate at 150-MHz. Using standard printed circuit board copper clad FR4 material for the substrate, the resulting RMPA was about eighteen inches in diameter.

The physical orientation of the RMPA is important relative to the line of travel of the vehicle used over the tunnels. RMPA 202, 300, and 600, for example, radiates off only two of its edges, whether it is a square patch, a rectangular patch, or even a circular patch. Every RMPA has a feedpoint, and here the feedpoint connects to a phase detector and an impedance detector. The radiating edges of the patch are the ones that form “perpendicular” lines with the axis of the feedpoint through the physical center point of the patch. Drawing an imaginary line from the feedpoint to the center of the patch, the edges of the patch that are tangent to such line are the radiating edges. Radiation from these edges produces a linear polarization, and this is the axis for which phase affects are the most pronounced. RMPA's 202, 300, and 600 are positioned such that their movement relative to tunnel 206 is longitudinal to the imaginary line drawn from the feedpoint to the center of the patch. Such radiating edges are turned to be perpendicular to the expected directions of people travel. Inside a pipe, that can only be along the inside of the pipe. Two RMPA's set at right angles to one another could be used in spaces not so restricted.

Scattering parameters (s-parameters) describe the scattering and reflection of traveling waves when a network is inserted into a transmission line. Here, the transmission line is the air gap 604, the earth 601, and the air inside tunnel 606. S-parameters are normally used to characterize high frequency networks, and are measured as a function of frequency. See, National Instruments explanation at, zone.ni.com/devzone/cda/tut/p/id/2513. So frequency is implied and complex gain and phase assumed. The incident waves are designated by the letter a_(n), where n is the port number of the network. For each port, the incident (applied) and reflected waves are measured. The reflected wave is designed by b_(n), where n is the port number. When the incident wave travels through a network, its gain and phase are changed by the scattering parameter. For example, when wave a₁ travels through a network, the output value of the network is simply the value of the wave multiplied by the relevant S-parameter. S-parameters can be considered as the gain of the network, and the subscripts denote the port numbers. The ratio of the output of port-2 to the incident wave on port-1 is designated S₂₁. Likewise, for reflected waves, the signal comes in and out of the same port, hence the S-parameter for the input reflection is designated S₁₁.

For a two-port network with matched loads:

S₁₁ is the reflection coefficient of the input;

S₂₂ is the reflection coefficient of the output;

S₂₁ is the forward transmission gain; and

S₁₂ is the reverse transmission gain from the output to the input.

S-parameters can be converted to impedance by taking the ratio of (1+S₁₁) to (1−S₁₁) and multiplying the result by the characteristic impedance, e.g., 50-ohms or 75-ohms. A Smith chart can be used to convert between impedance and S-parameters.

The frequency and impedance, or reflection coefficient (S₁₁), of RMPA 202, 300, and 600 are measured to provide sensor information and interpretive reports. RMPA 202, 300, and 600 is electronically tuned by a sensor controller either adjusting oscillator frequency and/or varactors to find the resonant frequency of the RMPA each time a measurement is taken. The S₁₁ (reflection coefficient) parameter is measured in terms of magnitude. The sensor controller seeks to minimize the magnitude of S₁₁, meaning RMPA 202, 300, and 600 is near its resonant point and 50-ohms.

During an automatic steady state calibration, an iterative process is used in which sensor controller seeks a minimum in S₁₁ by adjusting the applied frequency. Once a frequency minimum for S₁₁ is found, sensor controller adjusts a bias voltage on varactors 121 and 122 connected to the edges of RMPA 202, 300, and 600. The voltage variable capacitances of varactors are used to fine tune RMPA 202, 300, and 600 into resonance, and this action helps drive the impedance as close to 50-ohms as possible. Sensor controller simply measures the S₁₁ magnitude minimum. Once voltage adjustments to varactors find a minimum in S₁₁ magnitude, the process is repeated with very fine adjustment steps in frequency control to find an even better minimum. The voltages to varactors are once again finely adjusted to optimize the minimum.

After calibration, an independent shift away from such minimum in S₁₁ magnitude means a tunnel passing underneath is affecting the balance. The reflection coefficient (S₁₁) will change away from the original “calibrated” resonance value. Typically a person passing within the field will cause a peak maximum in the measured data. The rate of change of the measured signal in the area is directly related to the speed of the person passing through the fields of lobes 125 and 126.

S₁₁ has both magnitude and phase, a real and imaginary part. Changes in magnitude indicate a disturbance in the EM-field of RMPA 202, 300, and 600, and changes in the phase provide the directionality of travel. RMPA is a linearly polarized antenna, the fields on one edge of RMPA are 180-degrees out of phase from the field on the other edge. With a proper alignment of RMPA in situ, tunnels passing under RMPA 202, 300, and 600 from front to back, for example in FIG. 1, will produce a phase signature. The phase at resonance can be corrected to provide a constant 180-degree shift.

The response versus distance is not linear, but the changes in RMPA electrical properties are linear with linear increases in bulk dielectric changes. In other words, with this, the system can be set to ignore changes below a certain threshold until the threshold associated with a human is detected.

Inventors Larry G. Stolarczyk and Gerald Stolarczyk described a practical way to implement some of electronics equipment that can be adapted for use here in the present invention. See, U.S. Pat. No. 6,633,252, issued Oct. 14, 2003, and titled Radar Plow Drillstring Steering.

FIG. 7 represents a computer program 700 in an embodiment of the present invention for a microcomputer that when executed performs the functions of the analyzer 216 in FIG. 2 and electronics 602 in FIGS. 6A-6C. Any changes in magnitude and phase of the input impedance (Z_(in)) of the RMPA that occur, e.g., tunnel 120 or 606 passing under the RMPA's of FIGS. 1 and 6C are analyzed. A subroutine 702 allows a user to set the minimum trigger threshold so that the system will not respond to small anomalies. A subroutine 704 uses the electronics disclosed in FIGS. 1-6C to obtain an input impedance (Z_(in)) of the RMPA. Such measurement include the magnitude and phase of the Z_(in). A subroutine 706 tests the magnitude of the Z_(in) to see if it exceeds a threshold magnitude change. If not, computer program 700 returns to subroutine 704 to take more measurements of Z_(in). If yes, a subroutine 708 analyzes the present and recent measurements of Z_(in) to see if the magnitude of Z_(in) is changing over a relatively short period of time, e.g., one second. If not, computer program 700 returns to subroutine 704 to take more measurements of Z_(in). If yes, a subroutine 710 analyzes the present and recent measurements of Z_(in) to see if the phase of Z_(in) is changing over a relatively short period of time, e.g., one second. If not, computer program 700 returns to subroutine 704 to take more measurements of Z_(in). If yes, a subroutine 712 reports or annunciates to a local or remote user that a tunnel has passed under the RMPA.

A subroutine 714 further reports an angle relative to the RMPA. The angle is ascertained from the phase measurements of Z_(in), and how they correlate to magnitude measurements at the same instants. The computer program 700 then loops around to subroutine 704 to take more measurements of Z_(in). Each measurement of Z_(in) is obtained while the RMPA is in resonance. AFC 210 (FIG. 2) continually hunts for the optimal frequency that will produce a minimum Z_(in). The minimum obtainable Z_(in) will change as tunnels pass under RMPA 202, 300, and 600.

Angle information can be used with tomographic calculations to improve tunnel detection and location by increasing the number of data points contributing to the analysis.

A method embodiment of the present invention for detecting any underground passageways includes radiating the earth and an underground passageway with a resonant microwave patch antenna (RMPA). The frequency of a continuous wave (CW) transmission applied to the RMPA in real-time is maintained to match the resonant frequency of the RMPA as it changes with variations in the loading caused by the earth and any underground passageways. Each minimum in the instantaneous input impedance of the RMPA is interpreted as an indication of its being operated at its resonant frequency given how it is loaded. Underground passageways are announced after analyzing how the instantaneous input impedance of the RMPA changes with changes in position of the RMPA on the surface in a local area. The relative direction of the underground passageway can be further determined by analyzing how the phase angle of the instantaneous input impedance of the RMPA changes.

Although the present invention has been described in terms of the presently preferred SPS receivers, it is to be understood that the disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the “true” spirit and scope of the invention. 

What is claimed is:
 1. A tunnel detection system, comprising: a resonant microwave patch antenna (RMPA); a vehicle on which the RMPA is mounted and pointed down toward the earth, and capable of rolling over the surface of the earth; an automatic frequency control (AFC) connected to the RMPA and providing for a continuous wave (CW) transmission signal that is constantly and automatically tuned to the resonant frequency of the RMPA; an impedance measurement device configured for continuous measurements of the input impedance (Z_(in)) of the RMPA; an interpretive device connected to receive impedance measurements, and configured to analyze and report short-term changes in the magnitude and phase angles of individual impedance measurements of the RMPA as the passing of the vehicle and the RMPA over a tunnel in the earth.
 2. The tunnel detection system of claim 1, further comprising: a mapping device for marking a map with said reports from the interpretive device and for marking said map with a current location of the vehicle.
 3. The movement detection system of claim 2, wherein the sensor controller controls the oscillator such that the microwave antenna is continually in resonant operation.
 4. The movement detection system of claim 1, further comprising: at least one varactor connected to the microwave antenna and useful for fine tuning the resonant frequency of the microwave antenna.
 5. A tunnel detection system, comprising: a resonant microwave patch antenna (RMPA) for mounting on a surface vehicle, wherein the RMPA reacts differently to dielectric changes in its fringing and propagating e-fields according to relative direction; a sensor controller connected to the RMPA to drive the RMPA with a variable frequency, and to measure the impedance and phase (Z_(in)) of the RMPA while keeping the RMPA at frequency resonance; and an interpretive device that interprets particular changes in the resonant frequency, impedance, and phase of the RMPA as being the passing of tunnels underneath; wherein, buried air voids near the RMPA affect the dielectric constant of any intervening air gap, and thereby also affect the resonant frequency, impedance, and phase of the RMPA.
 6. The tunnel detection system of claim 5, wherein a phase change is indicative of an angle of a tunnel relative to the RMPA, and a delta frequency and impedance change indicates the size and proximity of the tunnel to the RMPA.
 7. The tunnel detection system of claim 5, further comprising: a threshold detector to screen out any false positives; wherein, an interpretation of changes in resonant frequency, impedance, or phase of Z_(in), is output to annunciate the locating of a tunnel.
 8. The tunnel detection system of claim 5, further comprising: a device for using angle information with tomographic calculations to improve tunnel detection and location by increasing the number of data points contributing to each analysis.
 9. A method for detecting underground passageways, comprising: radiating the earth and any underground passageways with a resonant microwave patch antenna (RMPA): maintaining the frequency of a continuous wave (CW) transmission applied to the RMPA in real-time to match the resonant frequency of the RMPA as it changes with variations in the loading caused by said earth and any underground passageways; interpreting a minimum in the instantaneous input impedance of the RMPA as an indication of its being operated at its resonant frequency given how it is loaded; and announcing the presence of an underground passageway by analyzing how the instantaneous input impedance of the RMPA changes with its changing position on the surface.
 10. The method of claim 9, further comprising: announcing the relative direction of an underground passageway by analyzing how the phase angle of the instantaneous input impedance of the RMPA changes with its changing position on the surface. 